The present invention relates to polycarbosilanes containing a hetero element and to a process for preparing these hetero element-containing polycarbosilanes.
It is known in the prior art that polycarbosilanes and polycarbosilane derivatives can be used, inter alia, as polymer precursors for producing ceramic fibers by the so-called polymer precursor method. The polycarbosilane polymers are first prepared for this purpose. Such polycarbosilanes are polymers having a skeleton structure consisting of the elements carbon and silicon, the polymers being composed of a skeleton having a backbone of (--Si--CH.sub.2 --) units, in which two side chain groups are bonded to every Si atom. The polycarbosilanes are then spun, for example, to give polymer fibers, which can be converted into a ceramic SiC fibers (for example Nicalon) by pyrolysis processes in a non-oxidizing atmosphere. To be useful as precursors for pyrolytic preparation of a ceramic fiber, it must be possible to convert the polycarbosilanes into the ceramic fiber with high yields while maintaining the original polymer morphology, i.e. while maintaining a fiber form of which the polymer is a model. Often, however, instead of the desired SiC formation, melting processes, decomposition reactions which form large volatile molecules, shrinking processes, and weight loss are also observed to a greater or lesser degree during production of the fibers, depending on the nature of the polycarbosilane employed. The properties of the resulting SiC products are adversely influenced by such side reactions, and therefore the products often prove to be unusable.
In order to counteract the aforementioned disadvantages, in the prior art on the one hand hetero atom compounds, such as, for example, polyborodiphenylsiloxane (PBDPSO) and polymetallo-siloxanes, i.e. hetero element compounds which contain metalloxane units --Mt--O--(Mt=metal, for example Ti or Zr) and siloxane units --Si--O--, are added to the polycarbosilanes as sintering aids during pyrolysis to produce fibers. On the other hand, hetero elements such as, for example, B, Ti, Zr, Cr, Mo and others, are incorporated into the polycarbosilane skeleton as metalloxane units --Mt--O--. These polymetallocarbosilanes of the prior art are thus derived from polycarbosilanes, the main backbone chain of which is essentially formed from structural units --Si--CH.sub.2 -- and metalloxane units --Mt--O--. Polymetallocarbosilanes such as, for example polytitanocarbosilane, are also added to polycarbosilanes as binding and sintering additives for further processing by pyrolysis.
The hetero element in the polymetallocarbosilanes and . polymetallocarbosiloxanes, as in polyborodiphenylsiloxane, is always bonded to the silicon atoms of the polycarbosilane via oxygen atoms. Consequently, only ceramic fibers which likewise necessarily always contain oxygen can be prepared from these polycarbosilane derivatives containing a hetero element. However, to improve the properties of the ceramic fibers, it is often desirable to adjust the content of oxygen in the ceramic fiber to as low a value as possible. There is therefore a need for polycarbosilanes which contain a hetero element without any oxygen, so that the hetero element is incorporated into the polycarbosilane skeleton without oxygen bridges.